This invention relates to acoustic well logging in general and more particularly to methods and apparatus for generating and detecting acoustic waves in a formation, particularly of the acoustic shear wave type.
It has long been known in the investigation of sub-surface earth formations traversed by a borehole that measurements or "logs" of acoustic energy introduced into the formation can yield extremely useful information about various formation parameters and characteristics. Accordingly, it has been conventional to introduce a logging sonde into the borehole containing some form of acoustic wave generator and receiver, to direct acoustic energy from the generator into the formation adjacent the borehole elevation of interest, and to thereafter record with the receiver the resultant acoustic waves returning from the formation.
One acoustic wave of particular interest is known in the art as the "shear" or "S" wave, which may develop in a formation as a result of vibratory motion in the formation at right angles to the direction of travel of the wave. A general discussion of this and related "compressional" (or "pressure") wave phenomena may be found in "The Full Acoustic Wave Train In A Laboratory Model Of A Borehole" by S. T. Chen, Geophysics, Volume 47, No. 11, November 1982, and in the aforementioned patent applications, all of which are herewith incorporated by reference for all purposes.
Shear wave logging has become increasingly useful in the detection of formation fractures as well as in determination of lithological properties of formations and the like. However, several problems have contributed to the difficulty in successful usage of this technique.
For example, often it has been found that the amplitude of the shear wave is insufficient for effective processing and analysis. Typically, the shear wave requires a greater travel time than the compressional wave to traverse the longitudinal distance through the formation between the acoustic generator and the detector. Accordingly, it was often found difficult to discriminate between this first-arriving compressional wave and the later-arriving shear wave (which may arrive before the compressional wave has completely attenuated.
Attempts were made to increase the magnitude of the S wave impinging upon the formation in order to increase the magnitude of the received S wave relative to the other signals, thereby increasing the signal to noise ratio. Such research produced some useful results, such as the realization that the angle at which the acoustic energy was introduced into the formation could enhance the formation of S waves, and the further discovery that multipole acoustic sources, such as quadrupole sources (discussed in aforementioned U.S. patent application Ser. No. 379,684), could more effectively produce desired S waves and provide a means for direct S-wave logging. The expression "multipole source" is used herein to denote sources of dipole, quadropole or higher order acoustic waves; but not to denote axially symmetric monopole sources.
However, severe problems still remained in the successful production of such S waves. For example, it has been known that multipole sources are less efficient acoustic radiators than are monopole sources. Accordingly, to obtain the benefits of multipole sources for direct S-wave logging, with improved signal to noise ratios over the compressional wave "noise" and other noise, more powerful multipole order sources were required.
Several design constraints were presented which hampered the creation of more powerful S wave sources. In particular, for conducting acoustic S-wave logging operations in soft formations it was often necessary to provide strong sources of S waves having frequencies less than three KHz. This, in turn, generally suggested physically large sources to obtain the necessary low resonant frequencies. However, use of large high-voltage source power supplies to energize such physically large sources was disadvantageous due to the attendant need for complicated electric circuit design and due to high voltage noise interference problems associated with such high voltage supplies.